As communication technology advances, more methods of communication become available to consumers. Miniaturization of electronic components and a concomitant reduction in cost have enabled a large number of consumers to afford cutting-edge telecommunication technology, for example, cellular telephones. Continuing the relentless drive to improve transmission speed and reception quality, a number of cellular telephone protocols and short-range wireless communication protocols have emerged.
One issue facing the wireless communications is that different wireless protocols require different frequencies and bandwidths. For example, the PDC system in Japan operates at 800 MHz and has a bandwidth of only 30 kHz, whereas 802.11 a operates at 5.4 GHz with a 20 MHz bandwidth. There are thus nearly a factor of ten in the range of common operating frequencies and a factor of thousand in the range of operating bandwidths. There is therefore a need for a small and inexpensive wireless communication device that can operate at wide ranges of operating frequencies and operating bandwidths.
Microstrip antennas, however, are not suitable for wireless communications in such wide operating frequencies and operating bandwidths. Typical microstrip antennas suffer from several drawbacks: first, they only operate at a single frequency. Second, they have relatively narrow bandwidth. Third, they do not have good gain performance. It is known that multi-band antennas can simultaneously serve as antennas for AM/FM broadcast radio and for Citizen Band transceivers. As discussed in USPN U.S. Pat. No. 6,107,972 to Seward, et al., one problem in designing antennas of this type is to define an antenna that has near optimal receiving/transmission capabilities in several separate frequency bands. The AM radio band falls in the comparatively low frequency range of 550 to 1600 KHz while FM radio operates in the 88 to 108 MHz range and CB operates in the relatively narrow range of 26.95 to 27.405 MHz. Cellular telephone operates in a frequency band of 825 to 890 MHz. Basic antenna design principles dictate that a commonly used electrical length for a rod antenna used with a ground plane is one-quarter of the wavelength of the transmitted signal. Thus, there is a design conflict when a single antenna is used for several frequency ranges. One option used in prior art antenna design is to tune the antenna to the separate frequencies when switching between bands. This has obvious disadvantages to the user of the radio, using impedance matching networks. Another option is to design an antenna that provides a compromise and is usable in several frequency bands. Such an antenna, by its nature, provides near optimal reception in at most one frequency range. For example, it is not uncommon in automobile antennas to use an antenna length equivalent to one-quarter wavelength to the midpoint of the FM range. As a consequence, the lower frequency AM reception is not optimum but is acceptable. However, such an antenna is unacceptable for use with a cellular or CB transceiver. In vehicles, it is common to use one antenna for CB, another for AM/FM, and a third for cellular telephone.
A major challenge for all wireless devices communicating in different operating frequencies and bandwidths is to control the manufacturing cost and the device size. Ideally, the antenna should be manufactured at lower than one dollar in cost for portable devices. If multiple antennas are required for different bands, it would be difficult to meet this cost goal or to fit all the antennas on a single Network Interface Card or a Compact Flash Card.